The invention relates to a process for the recovery of waste gas from a coke oven operation.
In a coke oven there is effected an indirect heating of the coal charge after it has been introduced into the heating chambers. The heat which is necessary for the coking operation is generated by burning heating gases in heating ducts which are formed by the walls of the coke oven chambers with rectangularly arranged heater girders. The heat is transferred by radiation and convection from the flame and reaction products to the walls of the oven chambers. By ducts it is then passed to the inside of the chambers from where it is again principally transferred by ducts, and only to a small portion by convection through the gasified products as heat carrier media, and by solid body radiation. This kind of heating operation because of the high terminal coking temperatures of about 1000.degree. to 1200.degree. C. results in high waste gas temperatures.
The recovery of heat contained in the waste gases is carried out either in periodically operating regenerators or in continuously operating recuperators. In these two types of apparatus a preheating of the heating air and possibly also of the fuel gases takes place. These parts of the apparatus because of the high temperatures are built from ceramic materials. The heat transfer in the ceramic heat storage material of a regenerator is effected by conduction and likewise, in case of a recuperator, by means of the separating walls between the heat exchange materials, i.e. the waste gas and the air.
With ceramic regenerators or recuperators it is largely possible in coke ovens to recover the waste gas and to reintroduce it into the fuel process. A disadvantage is that because of the necessity of using ceramic materials the investment costs are high. The regenerators or recuperators must have a substantial height of construction in order to obtain a sufficient heat recovery. The use of ceramic materials for the regenerators or recuperators because of the difficult sealing properties, does not permit substantial pressure differences on the gas-side of the apparatus. The result is that the heat exchanging gases can be caused to flow only with a low to moderate flow speed. The leaks which would occur in case of pressure differences between the heat exchanging media would otherwise constitute a source of malfunction and would considerably lower the effective degree of the combustion.
The heat transfer could be theoretically based principally on the radiation of the waste gas components--steam and carbon dioxide--on the waste gas side. With this form of heat transfer the ceramic material of the separating walls of the storage materials reduces the heat flow substantially because of the high conduction resistance of these materials. Besides, the spaces in which the radiation substantially determines the heat passage must be built for reasons of space economies in a manner that substantial width for the gas radiation is not available and the intensity of the radiation can therefore not be fully utilized.
Necessarily this leads to a shift of the heat transfer by radiation to a heat transfer by convection which latter has lower heat transfer coefficients than the gas radiation. The results are again still more massive constructions and higher costs.
The heat transfer coefficient in the low temperature area on the waste gas side could be increased by increasing the turbulence since this results in the transfer of the convection heat. However, this can be accomplished only by an increase of the flow speed. The ceramic material of the conventional coke ovens, however, precludes this kind of increase for the above reason of leakage danger.
In the conventional coke ovens which are equipped with ceramic regenerators or recuperators there occurs a comparatively high air excess because of the flow conditions. This lowers the heating efficiency. What would be desirable would be a nearly stoichiometric conversion of fuel gas and air oxygen which would result in a lower oxygen contents of the waste gases and therefore in an optimum heating efficiency. Conventional coke ovens however do not permit this kind of fuel adjustment for the reasons given above.
It is therefore an object of the invention to improve the recovery of the waste gases from a coke oven operation and to propose a suitable process and apparatus for this purpose.